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In the illustrated out-of-order execution scheme without a reorder buffer, a reservation station identifier acts as the producer tag for an outstanding result. A register renaming map and waiting operands in other reservation stations can refer to that tag. The tag must remain unique until the result has been broadcast and the required register file update has occurred.

For example, issue a slow multiplication producing r1 into reservation station S, then an addition r4 := r1 + r5 into another reservation station. The addition records S as the unresolved source of r1. Even after the multiplier has accepted its operands, the multiplication can take many cycles; S cannot be reassigned merely because its operands have left the reservation station. If an independent short instruction is assigned S and broadcasts first, the addition would mistake that unrelated value for the multiplication's result. A later multiplication result could likewise corrupt the new instruction's register renaming mapping.

Finishing execution is also insufficient if the result is waiting for common data bus access: consumers and the register file still name S until writeback. Keep the producer tag allocated through broadcast/writeback, so no younger result can masquerade as the outstanding producer. With a separate persistent producer tag, such as a reorder buffer entry or a physical register, a design can release the scheduling slot earlier; this changes the tag-ownership arrangement in the question.